Patients with OSA have recurrent apnoeas or hypopnoeas during sleep that are only terminated by the patient arousing. A hypopnea may be considered a partial reduction in breathing that lasts at least 10 seconds during sleep. The 1999 American Academy of Sleep Medicine (AASM) guidelines specify a hypopnea condition as a 10 second reduction in flow of 50% or more or a reduction of flow of less than 50% followed by an arousal or a desaturation of at least 3%. Subsequent AASM guidelines specify a 30% reduction in ventilation and a desaturation of at least 4% as a hypopnea condition. These recurrent events cause sleep fragmentation and stimulation of the sympathetic nervous system. This can have severe consequences for the patient including day-time sleepiness (with the attendant possibility of motor-vehicle accidents), poor mentation, memory problems, depression and hypertension. Patients with OSA are also likely to snore loudly, thus also disturbing their partner's sleep.
The best form of treatment for patients with OSA is constant positive airway pressure (CPAP) applied by a blower (compressor) via a connecting hose and mask. The positive pressure prevents collapse of the patient's airway during inspiration, thus preventing recurrent apnoeas or hypopnoeas and their sequelae. Such a respiratory treatment apparatus can function to supply the patient with a supply of clean breathable gas (usually air, with or without supplemental oxygen) at the therapeutic pressure or pressures, at appropriate times during the subject's breathing cycle.
Respiratory treatment apparatus typically include a flow generator, an air filter, a mask or cannula, an air delivery conduit connecting the flow generator to the mask, various sensors and a microprocessor-based controller. The flow generator may include a servo-controlled motor and an impeller. The flow generator may also include a valve capable of discharging air to atmosphere as a means for altering the pressure delivered to the patient as an alternative to motor speed control. The sensors measure, amongst other things, motor speed, gas volumetric flow rate and outlet pressure, such as with a pressure transducer, flow sensor or the like. The apparatus may optionally include a humidifier and/or heater elements in the path of the air delivery circuit. The controller may include data storage capacity with or without integrated data retrieval/transfer and display functions.
To treat hypopnea, automated apparatus have been implemented with algorithms to detect a hypopnea condition based on data from a flow sensor. Examples of conventional apnea/hypopnea detection devices are taught in U.S. Pat. No. 5,295,490 to Dodakian; U.S. Pat. No. 5,605,151 to Lynn; U.S. Pat. No. 5,797,852 to Karakasoglu et al.; U.S. Pat. No. 5,961,447 to Raviv et al.; U.S. Pat. No. 6,142,950 to Allen et al.; U.S. Pat. No. 6,165,133 to Rapoport et al.; U.S. Pat. No. 6,368,287 to Hadas; U.S. Pat. No. 7,118,536 to Haberland et al. For example, in one such device, a hypopnoea can be deemed detected if a 10 or 12 second root mean square (RMS) of a flow (in L/sec) signal drops below 1.2 times the long-term average minute ventilation (in L/sec) (e.g., low pass filtering the absolute value of a flow signal divided in half), which approximately corresponds to a reduction to 50% of the normal RMS ventilation. A 50% reduction of an RMS ventilation may also be detected by comparing a ten second variance of a flow signal calculated over a 10 second window with the product of (0.5)2 and a sixty second variance of a flow signal calculated over a 60 second window. Such devices may also include a refractory period of time after detecting a hypopnea to prevent multiple scoring of a common hypopnea event. For example, the device may remain refractory for 15 seconds or until the RMS ventilation returns to at least three quarters of a long-term RMS ventilation for 15 contiguous seconds.
However, the question of the presence of a hypopnea condition itself is subject to interpretation. There is often considerable clinician variability in scoring the presence of a hypopnea. Thus, while automated apparatus may be implemented with various methods to detect the condition, differences in the implemented detection criteria can result in different hypopnea scoring for common breathing events.
It may be desirable to develop further methods for detecting hypopnea which may also be implemented in apparatus for detection and apparatus for treating upper respiratory conditions.